System and method for identifying the position of mobile terminals

ABSTRACT

The present invention relates to a system  10  and method for identifying the position of cellular telephones  12  in a geographical area served by a mobile telephony network  14.  The system  10  comprises a mobile location center  15  capable of analyzing information about the electromagnetic (RF) field transmitted by cellular telephones  12  with reference information including both RF information and probabilistic information associated with the morphology of the geographical area. The method involves assigning different weights to the location of cellular telephones  12  as a function of probabilistic reference information, and thus makes it possible to use statistical methods for resolving situations of ambiguity in locating cellular telephones which would otherwise be resolved in purely random fashion.

FIELD OF THE INVENTION

The present invention relates to a system and method for identifying theposition of mobile terminals or cellular telephones.

More particularly, the present invention relates to a system and methodfor identifying the position of a cellular telephone in a geographicalarea served by a mobile telephony network.

BACKGROUND OF THE INVENTION

Systems and methods for identifying the position of cellular telephonesare known in the prior art.

For example, International Patent Publication No. WO0018148 describes amethod for locating cellular telephones.

The prior art method makes it possible to locate cellular telephones bycomparing the Radio Frequency information collected from the cellulartelephone (cellular telephone's RF fingerprint or RF measurements) withRF information contained in a reference database (reference RFfingerprint), in which each reference RF fingerprint corresponds on aone-to-one basis with an elementary area or pixel of the geographicalarea served by the network, and by assigning the cellular telephone tothe position corresponding to the reference RF fingerprint whose valuesare closest to those of the cellular telephone.

While the prior art method would appear to provide reliable results fromthe technical standpoint, from the practical standpoint the results thatthis method furnishes are in general unreliable or inaccurate at best.

One practical problem is associated with the limited number of frequencychannels (channels) that cellular telephones use in order tocommunicate.

As the number of channels is limited (In Italy, for example, GSM (GlobalSystem for Mobile Communications) networks provide 124 channels, only aportion of which are allocated to each operator, it is usual for anoperator to assign identical channels to radio base stations that“cover” a given geographical area.

Because of this practice, which is also called frequency reuse, thecellular phone receives RF signals for each channel which correspond tothe algebraic sum, in terms of power, of the field values received fromthe respective radio base stations using the same channel. Consequently,the cellular telephone's RF fingerprint for a given channel may be thesame even when it is generated in different parts of the geographicalarea, given that it derives from a sum. As can readily be understood,this problem would not exist if the channels were not reused. In such acase, in fact, the field value for each channel would depend only onelectromagnetic loss factors due to the route taken to reach thecellular telephone.

Another practical problem springs from the fact that the cellulartelephones generate an RF fingerprint which includes the field values ofa limited number of channels. In the case of GSM, for example, thecellular telephones are capable of generating an RF fingerprint thatincludes a maximum of seven values (value septuplet), each correspondingto a field RF value for a different channel.

Consequently, it is extremely probable in practice that cellulartelephones positioned at different elements or pixels in thegeographical area generate identical septuplets, precisely because ofthe limited number of values that can be used. Naturally, when thecellular telephones can provide a number of values equal to the numberof available channels, the potential for error would be very limited.

A third technical problem is associated with the fact that the referencedatabases, even if updated regularly, contain field values which,regardless of the method used to obtain them, cannot correspond exactlyto the cellular telephones' field values because of the continualvariations in environmental and/or weather conditions which determinethe latter values.

For this reason, it cannot be realistically assumed in practice that thecellular telephone's position is uniquely identified by a singlereference fingerprint, except in particularly fortunate circumstances.

Essentially, then, though the prior art method may be accurate intheory, it has been found in practice that this method results inlocation errors which in many cases may be significant because offrequency reuse, the limited number of values that the cellulartelephone uses to generate the RF fingerprint, and the fact that thevalues measured by the cellular telephone vary over time.

In GSM networks, for example, it has been determined experimentally thatthere is a systematical ambiguity in establishing cellular telephonelocation because the prior art method assigns the same septuplet topoints that are geographically quite distant, and may be up to severalkilometers apart.

DISCLOSURE OF THE INVENTION

The object of the present invention is a system and method foridentifying the position of cellular telephones which does not have thedrawbacks of the prior art method, even though it does not require thatthe characteristics of the cellular telephones and the information theygenerate in order to permit location be modified.

This object is achieved by the cellular telephone location system andmethod as described in the independent claims.

In particular, in accordance with the present invention, locationambiguity can be reduced by introducing additional probabilisticinformation to the reference database in order to assign cellulartelephone location statistically.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will be betterunderstood from the following description of a preferred embodiment ofthe invention, which is intended purely by way of an example and is notto be construed as limiting, taken in conjunction with the accompanyingdrawings, where:

FIG. 1 represents a block diagram of the system in accordance with theinvention;

FIG. 2 represents a logic diagram of the cellular telephones as shown inFIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

With reference to FIG. 1, the location system 10 in accordance with thepresent invention comprises a multiplicity of mobile terminals 12, e.g.,cellular telephones, randomly distributed over a given geographical areaand whose position is to be identified, a mobile telephony or cellularnetwork (network) 14, e.g., a GSM network, and a Mobile Location Center(MLC center) 15.

Each cellular telephone 12 (FIG. 1 and FIG. 2) is of known type andcomprises a radio frequency circuit (RF circuit) 22, a control circuit25 connected to the RF circuit 22 and capable of controlling theactivities of the cellular telephone 12 on the basis of programs storedin the control circuit 25, and a SIM card (SIM) 27, connected inaccordance with the prior art to the control circuit 25.

The SIM card 27, of known type, comprises programs which areappropriately prepared by a telephone operator, for example, to manageand bill telephone traffic or, as will be described in detail below, toenable the cellular telephone 12 to be located.

The control circuit 25, as is known, is capable in particular ofmeasuring, periodically and by means of the RF circuit 22,electromagnetic field values (RF measurements) in a given number offrequency channels (channels), and of selecting among these channels amaximum number of RF measurements, e.g., up to 7 for GSM networks,corresponding to an equal number of channels for which the cellulartelephone 12 is capable of decoding an associated identification code.

In accordance with the present invention, the SIM card 27 is capable inparticular of responding to appropriate commands set up by the telephoneoperator and activated by the cellular telephone user by causing thecontrol circuit 25 to perform a predetermined number of measurements,transferring the measurements to the SIM card, and transmitting them inthe form, for example, of SMS short messages.

The network 14, of known type, e.g. the GSM network, is capable ofreceiving the RF measurements transmitted by the cellular telephones 12and of transmitting them to the MLC center 15.

In general, the network 14 comprises a multiplicity of radio basestations (RB stations) which are not shown in the figure, and is capableof permitting the exchange of messages and communications betweencellular telephones 12 present in elementary points or pixels of thegeographical area and service centers, systems and equipment connectedto the network 14 and, for example, the MLC center 15.

The MLC center 15 comprises a computer 55 of known type, for example adual CPU Pentium® III computer with 512 Mbytes RAM and Windows® NToperating system, and a disk sub-system (disks) 52 of known type,connected to computer 55 and capable of storing reference databases in afirst memory zone 52 a, and modules or programs to be used inidentifying the position of cellular telephones 12 in a second memoryzone 52 b.

As will be described in detail below, the MLC center 15 is capable ofrunning the programs stored in zone 52 b and of identifying, by means ofthese programs and on the basis of the SMS messages received fromcellular telephone 12 and of the reference databases stored in zone 52a, the position of the cellular telephones and of transmitting theposition information so obtained to service centers and/or to thecellular telephones 12 by means of the network 14.

In accordance with a characteristic feature of the present invention,the reference database comprises a multiplicity of information strings(records), each consisting of information or fields having the meaningshown in Table 1 below:

TABLE 1 INFORMATION OR FIELDS I II to (N-1) No. Identifier RF fieldvalues for the Probability ρ_(p) for channels allocated to the that thegeographical telephone operator who manages cellular area pixel the MLCcenter and which telephone is p_(i). identify the pixel through alocated in the one-to-one correspondence. pixel.

In particular, the number of information items relating to RF fieldvalues correspond, for example, to the number of frequency channelsallocated to the telephone operator and is larger than the number ofchannels that cellular telephone 12 is capable of decoding.

In addition, the probability or factor ρ_(p) that the cellular telephoneis located in pixel p_(i) or traffic probability ρ_(p) is determined ona pixel-by-pixel basis using cartographic information such as thefollowing:

-   -   Morphological classification of the pixel;    -   buildings Percentage on the pixel;    -   Presence of communication infrastructures such as roads,        highways,    -   railroads, etc.

In accordance with this example of a preferred embodiment of theinvention, the altitude above sea level of each pixel was not regardedas contributing to determining factor ρ_(p) more reliably, and was thusnot considered as an active element in defining this parameter.

To facilitate the normalization operation without limiting the extent towhich the formal approach taken to the problem is generally applicable,a range of values from 0 to 1 was assumed for factor ρ_(p), where thetraffic that can be expected for pixel p_(i) increases as ρ_(p)increases.

In accordance with this example of a preferred embodiment of theinvention, it is assumed that a finite number of levels (i.e., a totalof 10) will be used for ρ_(p). The method used to assign these levels asa function of geographical characteristics is presented in the diagramshown in Table 2 below.

As will be readily apparent to a person skilled in the art, thedefinition of the values assumed by parameter ρ_(p) depends ongeographical characteristics in accordance with relationships which, incertain cases, combine various attributes through logical “or” and/or“and” operations. For example, the condition for the first level(ρ_(p)=0.1) is, first of all, that the corresponding pixel is covered bya buildings percentage equal to 2% and (“and”) that one of the followingthree (“or”) conditions be satisfied: bare area, glacier, dense forest,and (“and”), secondly, that there must be no communicationinfrastructures (no main road and no major highway).

It is clear on the whole that a pixel which satisfies these conditionsis intuitively at the bottom of the scale of potential traffic values.Likewise, it should be borne in mind that the intrinsic numerical valueof 0.1 is to be understood as relative to the overall range (0 to 1) andnot as an absolute value, for which no assumptions are made in theabsence of specific information.

Thus, the other values of ρ_(p) are defined by a unique assignmentmechanism which is readily implemented starting From the geographicalinformation available to a person skilled in the art.

Obviously, the diagram in Table 2 derives from a reasonable series ofassumptions, which are also dictated by considerations that can beverified experimentally through analyses of samples of the geographicalarea, and are intended to establish the average number of peopleoccupying the area and the composition/activities of this population.

TABLE 2 Factor ρ_(p) Morphological/land use categories 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9 60% < % covered by buildings ≦ 70% 1 % covered by buildings > 70%

As will be readily apparent to a person skilled in the art, the numberof discrete values that can be assigned to ρ_(p) and the techniques usedto define these values can be varied at will without departing from thegeneral criteria of quantizing the probability of traffic for thevarious pixels in the geographical areas as a function of cartographicparameters.

In accordance with the method contemplated by the present invention,operation of the system 10 described above is as follows.

When the user of the cellular telephone 12 activates the locationfunction, the SIM card 27 causes the control circuit 25 to activate oneor more measurements in sequence of the electromagnetic field in thechannels available to cellular telephone 12 by means of the RF circuit22.

The RF measurements are transferred by the control circuit 25 to the SIMcard 27 which, on the basis of the programs stored in its internalmemory, causes the control circuit 25 to transmit these measurements tothe MLC center 15.

The RF measurements thus transmitted relate, e.g., for the GSM standard,to a maximum of 7 field values for 7 channels.

By means of computer 55 and the programs and data stored in disks 52,the MLC center 25 compares the received field values with the fieldvalues of the respective channels stored in the records of the referencedatabase using a predetermined tolerance.

If the outcome of this comparison is negative, or in other words if norecord and corresponding pixel are found in the reference database, thecomputer 55 modifies the search tolerance in discrete steps in order toidentify at least one record corresponding to the received RFmeasurements in the reference database.

If comparison makes it possible to identify a single record, the searchends and the MLC center 25, by means of the computer 55 and network 14,transmits the location information to the cellular telephone and/or tothe service center set up to support queries associated with location.

If, as in fact invariably occurs in practice, the comparison identifiesa plurality of records, and hence of candidate pixels for establishingthe cellular telephone's location, the computer 55 takes the ρ_(p)factors from the records thus identified, assigns a weight proportionalto the identified values to these factors, and calculates the positionof the cellular telephone by means of a statistical procedure using theweighted ρ_(p) factors, so that the higher the value of ρ_(p), thehigher the probability of assigning the position to the correspondingpixel will be.

In detail, a real range, (1,100) for example, is defined. For thisrange, it is assumed that a random number generator with a uniformprobability density is available. Thus, in the presence of, say, tworecords, the first with ρ_(p=1)=0.8 and the second with ρ_(p=2)=0.2, thefirst pixel will be associated with a sub-range R1 of amplitude (1,80),while the second pixel is associated with a sub-range R2 of amplitude(81,100). Extracting a random number from range R with the aforesaidgenerator makes it possible to assign the cellular telephone's positionto one of the two pixels with a probability equal to the respectiveρ_(p) factors.

In cases of ambiguity, the computer 55 transmits, for example, theinformation regarding the cellular telephone's position and informationregarding possible alternative positions and the correspondingprobabilities to the service center.

Through the proposed method, it is thus possible to manage situations ofambiguity in weighted fashion, avoiding the problem which is frequentlyencountered in practice of having to assign a position at random to thecellular telephone in cases of ambiguity, without taking probabilisticfactors associated with the geographical area's cartographic parametersinto account.

As it has been found in practice that the number of possible positionswhich can be assigned to the cellular telephone in very largegeographical areas such as an Italian region, for example, is in theorder of several hundreds, it is also proposed in accordance with avariant to the present invention that the reference database include afurther field or information associated with each pixel p_(i) in thegeographical area, viz., the identifier of the RB station which isenabled to exchange information with the cellular telephones positionedin said pixels.

In accordance with this variant, the SMS message transmitted by thecellular telephone includes the RB station identifier as well as thefield values. Consequently, the computer first compares the cellulartelephone's RF measurements with the field values in the records havingthe same received RB station identifier.

Naturally, the method used to assign a position to the cellulartelephone by means of weighted probabilities ρ_(p) remains unchanged inthe proposed variant.

Though the foregoing description of a preferred embodiment of theinvention applies to a GSM network, it will be readily apparent to aperson skilled in the art that the method of assigning a probabilisticweight to the pixels in the geographical area can also be extended todifferent types of network where ambiguity in identifying the positionof cellular telephones can arise when only electromagnetic fieldmeasurements are used.

The circuitry and connections contemplated in the foregoing descriptionare capable of modifications in various obvious respects, as are thedetails of the operating method as illustrated, all without departingfrom the scope of the invention.

1. A system for identifying the position of a mobile terminal in ageographic area defined by a multiplicity of pixels and served by awireless telecommunications network having a plurality of radio basestations, the system comprising: the mobile terminal having means forcapturing a mobile-terminal fingerprint including field valuescorresponding to a first number of channels in the RF spectrum; andmeans for transmitting the mobile-terminal fingerprint; at least onereference station having means for capturing the mobile-terminalfingerprint and comparing it with a plurality of reference fingerprintseach corresponding to a position of the respective mobile terminal inthe network, the reference fingerprints each in turn comprising a) afield value corresponding to a second number of channels in the RFspectrum, the first number being a subset of the second number; and b) arespective probabilistic value associated with each pixel of the areaand indicative of the probability that the mobile terminal is located inthe respective pixel of the geographic area and associated withcartographic information for the geographic area representative of amobile-terminal traffic probability for the respective pixel; means forselecting a predetermined number of candidate fingerprints from theplurality of reference fingerprints on the basis of the comparisonbetween the plurality of reference fingerprints and the mobile-terminalfingerprint, the predetermined number of candidate fingerprintsidentifying a position of the mobile terminal in the geographic area;and means for assigning a unique position to the mobile terminal byattributing one of the candidate fingerprints to the mobile terminal onthe basis of the probabilistic value.
 2. The system in accordance withclaim 1 wherein the mobile-terminal fingerprint and the referencefingerprints also comprise: c) identifying codes for identifying a radiobase station capable of exchanging information with the mobile terminalin the geographic area.
 3. The system in accordance with claim 1 whereinthe reference station comprises means for transmitting the positionassigned to the mobile terminal to service centers connected to thenetwork.
 4. The system in accordance with claim 1 wherein the wirelesstelecommunication network is a GSM network.
 5. A method for identifyingthe position of a mobile terminal in a geographic area comprised of amultiplicity of pixels and served by a wireless telecommunicationsnetwork, the method comprising the steps of: capturing with the mobileterminal a mobile-terminal fingerprint including field valuescorresponding to a first number of channels in the RF spectrum;comparing the mobile-terminal fingerprint with a plurality of referencefingerprints each corresponding to a position in the network and eachcomprising a) a field value corresponding to a second number of channelsin the RF spectrum, the first number being a subset of the secondnumber; and b) a probabilistic value indicative of the probability thatthe mobile terminal is located in a respective pixel and associated withcartographic information for the geographic area representing of amobile-terminal probability for the respective pixel; selecting apredetermined number of candidate fingerprints from the plurality ofreference fingerprints and on the basis of the comparison between theplurality of reference fingerprints and the mobile-terminal fingerprintidentifying a possible position of the mobile terminal in the geographicarea; and assigning a unique position to the mobile terminal byattributing one of the candidate fingerprints to the mobile terminal onthe basis of the probabilistic value.
 6. The method in accordance withclaim 5 wherein the mobile-terminal fingerprint and the referencefingerprints also comprise: c) identifying codes for identifying a radiobase station capable of exchanging information with the mobile terminalin the geographic area.
 7. The method in accordance with claim 5,further comprising the step of: transmitting the unique positionassigned to the mobile terminal to service centers connected to thenetwork.